Wednesday, 24 February 2016

Zika Virus and Microcephaly

You have probably been as horrified and saddened as me to
see the shocking abnormality that affects newborn babies whose mothers have
been infected with the Zika virus.The
skulls and brains of the babies have not grown properly, and the babies appear
to have small heads, a condition known as "microcephaly".The standard definition is that the
circumference of the head is two (or three) standard deviations below average for
age and sex [1,2] (Fig. 1).

Origin and spread of
the Zika virus

Zika virus has been known since the 1940s, and originally
occurred in the equatorial regions of Africa.It is named after the Zika Forest near the Ugandan capital of
Entebbe.Analysis of the various
sequenced genomes has shown an origin in central Africa (a strain from Uganda
isolated in 1947 being the oldest), spreading elsewhere in Africa (Senegal
(1984), Nigeria (1968)and the Central
African Republic (1976)) and then spread westwards to Malaysia (1966), Cambodia
(2010), Micronesia (2007), French Polynesia (2013) and then Suriname and Brazil
(2015) [http://virological.org/t/initial-Zika-phylogeography/202].The virus is transmitted by mosquitoes such
as Aedes aegypti (Fig. 2) and A. albopictus.These mosquitoes are active during the day,
mainly at dawn and dusk and when the weather is cloudy, and transmit the virus
from patient to patient when the females take a blood meal.A.
aegypti is known as the yellow fever mosquito, and is particularly
distinctive with white rings around the leg joints and white markings on the
body.This mosquito originated in Africa
but has since spread throughout the tropics [3].There is also evidence that Zika virus can be
transmitted sexually via the semen of an infected man [4].

Zika fever, which has mild influenza-like symptoms, had been
thought to be a trivial disease.Now
there are a several questions that require answers.If there a causal link between microcephaly
and viral infection or are the symptoms coincidental?If the disease causes the symptoms, is this
an effect of viral enzymes, or a consequence of the body's own immunological
system attacking more than just the virus?

Microcephaly in
Brazil

Microcephaly is not a new condition, and can result from
chromosomal abnormalities as well as environmental conditions that can affect
brain growth.Mutations in the genes
MCPH1, which encodes the protein microcephalin, and ASPM, which encodes
abnormal spindle-like microcephaly-associated protein, can cause primary
microcephaly when the gene is homozygous [5- 7].Microcephaly is associated with other viral
diseases, such as chickenpox [8], but incidences are rare because women rarely
get the disease when pregnant because of the innate immunity they acquired
during childhood infection.It is
possible, of course, that the same may be true of Zika virus, which would
explain why microcephaly is not prevalent in Africa, because women acquire
immunity as girls, and would also explain the dramatic increase in the
condition in Brazil, where the disease arrived recently and pregnant women have
no immunity.The rates of Zika infection
and microcephaly in Brazil really are alarming.It has been estimated that 1.5 million cases of Zika fever occurred in
Brazil between April 2015 and January 2016, and 3718 cases of microcephaly (38
of which led to death) [9], which is one case per 403 infections, and one case
per 793 births (the population of Brazil is 204 million and the annual birth
rate is 14.46 per 1000 [https://www.cia.gov/library/publications/the-world-factbook/geos/br.html]).This is considerably higher than the known
incidence of microcephaly in the UK (where the Zika virus is absent):
approximately 1 in 10,000 births in the UK [http://www.rightdiagnosis.com/m/microcephaly/basics.htm].

Zika virus
polyprotein

The Zika virus is a flavivirus, a group that includes the
viruses that cause yellow fever, dengue fever, Japanese encephalitis and West
Nile fever.These viruses contain
single-stranded RNA as their genetic material, and the RNA encodes a single
polyprotein.This polyprotein consists
of several enzymes and structural proteins, and processing by an endogenous
serine endopeptidase is required to separate the individual proteins.By submitting the Zika virus polyprotein to
InterProScan, it is possible to identify all the components.These are shown below.There is no component with an unknown
function or one expected to affect brain development directly.

Fig. 3.Zika virus polyprotein domains identified by InterProScan.

How polyprotein processing progresses in the Zika vuris
polyprotein is unknown, but some of the cleavage sites have been mapped in both
the yellow fever virus and West Nile virus [10, 11].All known cleavages are performed by the
endogenous serine endopeptidase, but one cleavage can be performed by unrelated
host serine endopeptidases normally responsible for processing host protein
precursors [12].The specificity for
both the viral and host endopeptidases is similar: cleavage follows a pair of
basic residues (lysine or arginine) and precedes glycine, serine or
threonine.A pairwise alignment of the
West Nile and Zika virus polyprotein sequences, shows that the known cleavage
sites are conserved (Fig. 4).

Fig. 4 Conservation of polyprotein cleavage sites

Sites of cleavage are indicated by an arrow.Residues highlighted in pink are conserved
between West Nile virus (W Nile) and Zika virus.Residue numbers are shown above and below
each sequence.

60708090100↓110

W Nile
APTRAVLDRWRGVNKQTAMKHLLSFKKELGTLTSAINRRSTKQKKRGGTAGFTILLGLIA

:. ....:::.:. :.: : .:: ..::.:: :.:::::.:. ..:.:..

ZikaKPSTGLINRWGKVGKKEAIKILTKFKADVGTMLRIINNRKTK--KRGVETGI-VFLALLV

60708090100110

180190200210 ↓220230

W Nile
AAGNDPEDIDCWCTKSSVYVRYGRCTK--TRHSRRSRRSLTVQTHGESTLANKKGAWLDS

.:::.::::....... :: ::.: ..::::::.:. .:. . : .....::.:

ZikaEPQYEPEDVDCWCNSTAAWIVYGTCTHKTTGETRRSRRSITLPSHASQKLETRSSTWLES

180190200210220230

1370↓13801390140014101420

W Nile
DPNRKRGWPATEVMTAVGLMFAIVGGLAELDIDSMAIPMTIAGLMFVAFVISGKSTDMWI

..::.:: .::::::::. ::::::.. ::: ::
::.::. :..:.::::.::.:

ZikaTASKKRSWPPSEVMTAVGLICAIVGGLTKTDID-MAGPMAAIGLLVVSYVVSGKSVDMYI

137013801390140014101420

14901500↓ 1510152015301540

W Nile
ILPSVIGFW-ITLQYTKRGGVLWDTPSPKEYKKGDTTTGVYRIMTRGLLGSYQAGAGVMV

: : . . : . ..::.:..:: :::.: :::.::.:::::::: :::: :.:::::

ZikaI-PFAAAAWFVYIKSGKRSGAMWDIPSPREVKKGETTAGVYRIMTRKLLGSTQVGAGVMH

149015001510152015301540

2090210021102120↓21302140

W Nile
ITKLGERKILRPRWADARVYSDHQALKSFKDFASGKRS-QIGLVEVLGRMPEHFMGKTWE

::.::.:::.::: :::. :::
.:::::.::.:::.::.:..: .: :..:

ZikaWTKFGEKKILKPRWMDARICSDHASLKSFKEFAAGKRTIATGLIEAFGMLPGHMTERFQE

209021002110212021302140

25102520253025402550

↓

W Nile
HIMRGGWLSCLSITWTLIKNMEKPGL--KRGGAKGRTLGEVWKERLNHMTKEEFTRYRKE

.:.::..:.:. .:. .::.::::..:.:.:: ::::::.:::::..

ZikaNIFRGSYLAGPSLIYTVTRNA---GIMKKRGGGNGETVGEKWKERLNRMTALEFYAYKRS

250025102520253025402550

Is microcephalin a
substrate for the Zika virus endopeptidase?

Could it be that the viral endopeptidase is processing host
proteins at similar sites?There are at
least 24 human proteins known to be cleaved by viral endopeptidases.Cleaving eukaryotic translation initiation
factors and polyadenylate-binding protein 1 switches off the host cell's own
protein synthesis mechanism, ensuring that only viral proteins are made, and
the endopeptidases from retroviruses, enteroviruses and foot-and-mouth disease
virus all cleave these proteins [13-17].Nuclear pore glycoprotein p62 is also cleaved by the rhinovirus endopeptidase
picornain 2A peptidase, and this disrupts trafficking from the nucleus to the
cytoplasm [18].Both microcephalin
(http://www.uniprot.org/uniprot/Q8NEM0) and ASPM
(http://www.uniprot.org/uniprot/Q8IZT6) have regions that conform to the
specificity of the Zika virus endopeptidase (Fig. 5) so either could be a
potential substrate and be inactivated by cleavage.If cleavage of these proteins has the same
effect as mutations in the genes, then cleavage could lead to microcephaly.

Fig. 5 Potential cleavage sites in microcephalin and ASPM

MCPH166QSTWDKAQKR+GVKLVSVLWV

MCPH1
375PPKEKCKRKR+STRRSIMPRL

MCPH1
379KCKRKRSTRR+SIMPRLQLCR

MCPH1
467MSDFSCVGKK+TRTVDITNFT

MCPH1
486TAKTISSPRK+TGNGEGRATS

MCPH1
639LIKPHEELKK+SGRGKKPTRT

ASPM148NAEEQKKKKR+SLWDTIKKKK

ASPM243ATCLPLSVRR+STTYSSLHAS

ASPM431VPQSPEDWRK+SEVSPRIPEC

ASPM576TTASVARKRK+SDGSMEDANV

ASPM616SEPKTSAVKK+TKNVTTPISK

ASPM639NREKLNLKKK+TDLSIFRTPI

ASPM655RTPISKTNKR+TKPIIAVAQS

ASPM
1081FLKHTKSIKK+TISLLSCHSD

ASPM
1098HSDDLINKKK+GKRDSGSFEQ

ASPM
1584DRVRFLNLKK+TIIKFQAHVR

ASPM
2095QHKEYLNLKK+TAIKIQSVYR

ASPM
2184ASFRGVRVRR+TLRKMQTAAT

ASPM
2287MRRRFLSLKK+TAILIQRKYR

ASPM
2712RAKVDYETKK+TAIVVIQNYY

ASPM
3081ERIKYIEFKK+STVILQALVR

ASPM
3252IREENKLYKR+TALALHYLLT

Conclusions

The incidences of microcephaly in babies born to mothers
infected by the Zika virus in Brazil are not only alarmingly high, but much
higher than the background mutation rate that causes microcephaly in the UK;
there seems to be little doubt that the condition and Zika fever are
related.Whether this relationship is
because the disease is new to Brazil, mothers have no immunity and microcephaly
results from the body’s own immune response, as has been observed previously in
chickenpox, or because of the presence of a viral toxin, is not known.If the latter, then it is possible that the
proteins derived from genes in which mutations are known to cause microcephaly
are susceptible to digestion by the Zika virus polyprotein processing enzyme,
which is predicted to have a specificity similar to that of host prohormone
convertases: inactivating the proteins may have the same results as mutations
in the genes.Further research is
required to understand the mechanisms causing microcephaly, which might include
characterization of the viral endopeptidase.If the symptoms are due to the response of the immune system, then
microcephaly might be a transitory phenomenon, and once the population builds
up immunity, such incidences could become very rare in the future.

1 comment:

You mentioned that zika isn't present among africans because women build up an immunity to chickenpox when they are girls,the problem with that is chickenpox isn't prevelant in africa so why would they need to have immunity against it? Chickenpox is a form of hpv which again africans dont get unless they visit other parts of the world where the disease is prevelant.Also blacks lack the aspm gene,making it an impissibility for it to be manipulated by any virus.